Deciphering the Role of Hydrophobic Amino Acid l -Methionine in Tuning CO2 Hydrate Kinetics: Insights from Molecular Dynamics Simulations

Abstract

CO2 clathrate hydrate has emerged as a promising medium for carbon sequestration owing to its exceptionally high CO2 storage capacity and intrinsic stability in subsea environments. Sluggish growth kinetics hinder its field application and large-scale deployment. The hydrophobic amino acid, L-methionine (L-Met), has been identified as an efficient and environmentally benign promoter. However, the microscopic and concentration-dependent mechanisms governing its regulatory role remain elusive. Herein, we employ molecular dynamics simulations to unveil the nonmonotonic dependence of CO2 hydrate growth kinetics on L-Met concentration. Analysis on F4 order parameter and hydrate cages identifies a favorable L-Met concentration around 0.4 wt %, at which the CO2 hydrate growth rate is 60% higher and the CO2 storage capacity is 9% higher than that at ∼3.1 wt %. At optimal concentration, L-Met enhances both CO2 dissolution and diffusion, reducing the residence time of CO2 in nanoaggregates and in the aqueous phase by 8% and 13%, respectively. Potential of mean force (PMF) analysis indicates that the free-energy barrier for CO2 enclathration into hydrate cages is significantly reduced by 39% compared with pure water. In contrast, at elevated L-Met concentrations, despite the promotion of CO2 dissolution, CO2 molecules are strongly stabilized in solution, leading to an increased free-energy barrier of 33.2 kJ/mol for CO2 enclathration. In addition, hydrogen-bonding competition between L-Met and water further impedes CO2 hydrate growth at high L-Met concentrations. These findings establish a molecular-level framework for understanding the concentration-dependent tuning behavior of amino acids as gas hydrate kinetic promoters. The elucidated tuning mechanism provides theoretical guidance for the effective use of kinetic promoters in emerging CO2 hydrate-based technologies.